Japanese Patent No. 3811179 (hereinafter referred to as Patent Document 1) discloses a stator for a motor comprising a stator core including an annular yoke and a magnetic pole constituent member fitted inside the annular yoke, and a plurality of excitation winding portions. The magnetic pole constituent member includes a plurality of pole columns connected to the yoke, with the excitation winding portions wound therearound, a plurality of magnetic pole surface forming portions each including a magnetic pole surface, and a plurality of connecting sections each of which connects adjacent two magnetic pole surface forming portions. In an inner peripheral portion of the yoke, a plurality of fitting recessed portions are formed. The fitting recessed portions are open toward the magnetic pole constituent member and outer ends of the pole columns are fitted into the fitting recessed portions, respectively. In the inner peripheral portion of the yoke, a pair of or paired flat-surface portions, which are respectively continuous with an inner surface of a corresponding fitting recessed portion, are respectively formed on either side of the corresponding fitting recessed portion in a circumferential direction of the yoke. Each excitation winding portion includes a bobbin and a winding conductor wound around the bobbin. The bobbin is made of an insulating material and fitted with a corresponding pole column. This bobbin integrally includes a cylindrical portion that is fitted with the corresponding pole column, with the winding conductor wound therearound, a first flange portion that is provided at one end portion of the cylindrical portion and includes a pair of or paired flat portions which come into contact with the paired flat-surface portions of the yoke, and a second flange portion that is provided at the other end portion of the cylindrical portion and comes into contact with a corresponding magnetic pole surface forming portion.
In the stator core of such a type, in order to increase a space for the winding conductor wound around the cylindrical portion of the bobbin, it is necessary to reduce the thicknesses of the flange portions of the bobbin. In other words, in order to increase the volume which is occupied by the winding conductor wound around the cylindrical portion of the bobbin in the space within a slot, the thicknesses of the flange portions of the bobbin need to be reduced. However, when the thickness of the flange portions of the bobbin is reduced, the bobbin may be deformed or broken due to a stress when the winding conductor is wound. As a result, electrically insulating effect of the bobbin may be reduced, or the bobbin with the winding conductor wound therearound may not be able to be fitted with the corresponding pole column. Thus, in a conventional bobbin constituent member, there is a limit to reduction of the thicknesses of the flange portions of the bobbin.